Field of the Invention
The present invention relates to an exhaust passage structure of an outboard motor, in a multi-cylinder four-stroke-cycle engine mounted in the outboard motor, in which exhaust gas from exhaust ports of respective cylinder is discharged out of the engine.
Related Art
Prior art of an outboard motor of conventional art has provided an exhaust device, in which exhaust gases discharged from the exhaust ports of the respective cylinders of a multi-cylinder four-stroke-cycle engine mounted on an outboard motor are collected in an exhaust (gas) collecting portion (i.e., exhaust manifold) and then discharged out of the engine through catalyst portion, for example, see Japanese Patent Laid-open No. 2010-242744 and No. 2013-124594 (Patent Documents 1 and 2). In each of such documents, the exhaust collecting portion is formed to a cylinder head.
In the Patent Documents 1 and 2, as described above, the exhaust collecting portion for collecting exhaust (gas) discharged from the exhaust ports of the cylinder head is mounted to the cylinder head, and moreover, the exhaust collecting portion is formed with a guide surface parallel to a plane surface which passes cylinder axes of a plurality of cylinders formed to a cylinder block. According to such structure, the exhaust gas from the exhaust port collides with the guide surface of the exhaust collecting portion and flow direction of the exhaust gas is rapidly changed from a transverse direction to a perpendicular direction, which results in increasing in flow resistance of the exhaust gas, and hence, output of the engine is lowered.
In addition, in order to lower pressure of the exhaust gas colliding with the guide surface of the exhaust collecting portion formed to the cylinder head, it is necessary to separate the guide surface apart from the exhaust port. However, in such separate arrangement, the cylinder head projects outside in a width direction of the engine. As a result, the size or dimension of the engine is inevitably enlarged, and in a case where a plurality of outboard motors are mounted to a hull of a boat or like, a distance between the adjacently mounted outboard motors increases, which may apply adverse effect on operation or steering performance of the hull of the boat.